A conventional lock device for vehicle doors has a lock lever which can be moved between the locked and unlocked positions by means of the key cylinder, the inside locking button or the actuator. It is possible to change the known lock lever to the unlocked position by gripping the locking button with a gripping tool inserted into the clearance between the door and the vehicle body.
As shown in FIG. 21, U.S. Pat. No. 4,978,154 describes a lock device equipped with an anti-theft mechanism which can make such unjust operation as mentioned above impossible. The prior art lock device comprises a first lever A connected to the lock lever (not illustrated) and supported by a shaft B, a second lever C connected to the inside locking button D of the door and supported by the shaft B, an anti-theft member G movable in the left and right direction relative to the shaft B and rotatable integrally with the second lever C, said anti-theft member G having a projection F engaged with a forked portion E of the first lever A, and a changing member K having a slot H with which the projection F is engaged. Under the locked condition as shown in FIG. 21, when an inside nut M is caused to move left by a motor L, the changing member K begins to rotate to cause the anti-theft member G to move left, whereby the engagement between the projection F and the forked portion E is cancelled, and the anti-theft condition is produced. In the anti-theft condition, it is impossible to turn the first lever A to the unlocked position even though the anti-theft member G is turned clockwise by the locking button D.
The above prior art lock device has two major disadvantages. Firstly, there is doubt about the reliability of cancelling the anti-theft condition by a key cylinder. In cases where the lock lever is changed to the unlocked position by the key cylinder, the first lever A is displaced to the unlocked position in line therewith, then a slider N engaged with the first lever A moves right. However, as the inside nut M, which is in the anti-theft position, is not caused to move right even though the slider N moves right, the anti-theft member G is left at the anti-theft position. For this reason, it is understood that the prior art lock device is arranged so that the nut M is caused to move right by revolutions of the motor L when the lock lever is changed to the unlocked position by the key cylinder. However, in this case, if the battery power is completely consumed or the motor L malfunctions, an unexpected problem occurs.
The second disadvantage is that the first lever A is automatically changed to the unlocked position if the anti-theft conditon is cancelled by the motor L. In other words, it is impossible to cancel only the anti-theft conditon with the locked conditon maintained. Where public peace and order are not secured, if other doors are also unlocked when the driver's door is unlocked, a ruffian may get into a vehicle.